1. Field of the Invention
This invention relates to scanning systems for microarrays of biological species such as nucleic acids or proteins, and for any type of procedure or analysis where very rapid illumination, observation, and/or detection are performed at a large number of individual sites arranged in a spatial array. In particular, this invention relates to moving coil actuators and is of particular interest in connection with the use of such actuators as a driving mechanism for a mobile optical system.
2. Description of the Prior Art
Microarrays are two-dimensional arrays of sites where chemical or biochemical assays are performed, each site often being of microscopic dimensions, with an independent assay and often a different molecular species at each site. These arrays are formed on a variety of substrates, including glass slides, microtiter plates, and membranes. One of the widest uses of microarrays is in binding assays for the identification or characterization of an unknown biological species or the analysis of a sample to determine where the sample contains species with certain binding affinities. The size, number and spacing of the sites in a microarray can vary considerably, depending on nature of the individual species and the procedures to be performed. The sites can be the wells in a standard microtiter plate which has 96 wells in a 12×8 array with a spacing of 9 mm between wells. At the other extreme, the microarray can be formed on a single glass slide with a width of only 25 mm, where the sites are printed on by automated microprinting techniques. As many as 10,000 genes can be printed on a glass slide in this manner.
The monitoring of assay progress in a microarray and the detection of assay results often includes excitation by exposure of each site to incident radiation followed by, or simultaneous with, the detection and measurement of radiation emitted from the sites in response to the excitation. These functions are typically performed by an optical system with a scanning head, typically one equipped with a laser. Scanning systems often incorporate moving coil actuators to move the scanning head across the array, since these actuators can be operated at high speed, are simple and inexpensive to manufacture, can readily meet performance requirements, and are easy to control. The scanning head, which is the “payload” transported by the actuator, may include a mirror and lens assembly plus holders and bearings, or it may be a laser with detector optics. A moving coil actuator utilizes the well-known Lorentz force to drive the coil at high speed and with a high degree of control to meet precise specifications. The moving coil actuators that are commonly used for scanning heads are voice coil actuators, which are direct-drive, limited-motion devices that utilize a permanent magnetic field and an electric coil to produce a force proportional to the current applied to the coil. Examples of voice coil actuators in current use include various products from the Kimco Magnetics Divison of BEI Technologies, Inc., San Marcos, Calif., U.S.A., and from H2W Technologies, Inc., Valencia, Calif., U.S.A. Disclosures of voice coil actuators are found in U.S. Pat. Nos. 6,894,408, 6,870,285, 6,815,846, and 6,787,943.
For high-speed scanning, the moving parts must be limited to those of low inertia, and this is typically achieved by making the coil mobile and the magnet and magnet housing fixed. The length of travel of the coil is limited however, either by the length of the coil or the length of the magnet. As a result, the typical microarray scanner is only capable of scanning a width of about 1 inch (approximately 25 mm). The same scanner cannot be used effectively to scan a wider array such as that formed on a microtiter plate. Conversely, if an actuator is used for scanning an array whose width is narrower than the width for which the actuator is designed, either the coil will be traveling a greater distance than that needed for scanning the microarray or the system will require costly sensors and controls to limit the travel of the coil to a narrower width, with the added burden of additional mass.